Metapensiero-reactive

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- documentation updates.
- add a ``computation`` decorator.

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- require flush only when there are dependents.

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- fix a small bug in __delete__().

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- more tests.
- allow __set__ if generator is not defined.
- refactoring of Value's code.
- Fix behavior if Value's accessed when tracking isn't active.
- Provide a mechanism to halt tracking while computing if system
suspension is needed (``gevent``, ``asyncio``).
- Updates to the doc.
- Code is now tested in a pre-production environment.

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- small doc fixes.

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- Initial effort.
- Added testing with tox and ``gevent`` and ``asyncio``.
- Firt cut of the docs.